1. Field of the Invention
The present invention relates to a color ink deposition order determination method, image forming method, and image forming apparatus. In particular, the present invention are suitable to an inkjet printer that uses a plurality of colors of inks, and relates to a method for determining an order of depositing droplets of the inks, a method and an image forming apparatus for forming an image by depositing ink droplets in the determined deposition order.
2. Description of the Related Art
Many of the inkjet printers which are used in color printing form an image using a plurality of colors of inks including four colors of cyan (C), magenta (M), yellow (Y), and black (K). Regarding an order of depositing the plurality of colors of inks, Japanese Patent Application Publication No. 62-161541 discloses a method where ink with low brightness is firstly deposited. Japanese Patent Application Publication No. 2003-112469 discloses an inkjet recording method for using six pigment inks of black, light cyan, cyan, light magenta, magenta, and yellow, in which the pigment inks with high coloring power are deposited by priority.
In a shuttle scan type inkjet printer that causes a recording head to scan a predetermined image region more than once in order to record an image, ink droplets can be deposited by the shingling printing method so that the order of overlapped dots is changed. However, in a single pulse type inkjet printer that records an image by scanning once, the order of deposition of color inks (for example, C, M, and Y) is determined only by the order of disposed heads of the colors. For this reason, in the single pulse type printer, intended color density may not be obtained depending on the arrangement order of the heads of the color inks.
Such a problem is described with reference to FIGS. 21A and 21B as an example. FIGS. 21A and 21B are cross sectional views that schematically show a state in which ink is deposited on pixels A and B, which are adjacent to each other on a recording medium. In this case, it is considered that cyan (C) and magenta (M) inks are deposited on the pixel A and cyan (C) ink is deposited on the pixel B, in the predetermined orders including the order of C to M. Each dotted line in FIGS. 21A and 21B indicates the central position of each pixel, and diameter D indicates the dot diameter. In FIG. 21B, the diameter D of each dot is approximately three times greater than the distance between the pixels (pixel pitch Pp). FIGS. 21A and 21B show dots where droplets are deposited so as to overlap from bottom to top in the order of landing on the recording medium.
Specifically, FIG. 21A shows a state in which the inks are deposited in the order of C ink on the pixel A, M ink on the pixel A, and C ink on the pixel B (referred to as “deposition order 1”). FIG. 21B shows a state in which the inks are deposited in the order of C ink on the pixel A, C ink on the pixel B, and M ink on the pixel A (referred to as “deposition order 2”).
In the shuttle type printer, deposition orders of color inks can be controlled in units of recording pixels, and thus the shuttle type printer can adopt either patterns of the deposition order 1 shown in FIG. 21A or the deposition order 2 shown in FIG. 21B. In the single pulse type printer, on the other hand, a deposition order is uniquely determined by the arrangement order of the heads, and thus the single pulse type printer can adopt only the deposition order 2 shown in FIG. 21B of the deposition orders.
In the deposition order 2 shown in FIG. 21B, supposedly when the C ink has the properties of hardly reflecting (easily transmitting) light of the complementary color of M (i.e., light of green) and the M ink has the properties of easily reflecting (hardly transmitting) light of the complementary color of C (i.e., light of red), then the color of C can be almost invisible in a region Q (a region in which the uppermost layer contains the M ink) shown in FIG. 21B.
It should be noted that, for convenience of explanation hereinafter, “light of a complementary color of α” is described as “light of an α wavelength range”. For example, “light of an M wavelength range” means light of green.
A case where two colors of inks α and β (inks α and β are any of cyan ink, magenta ink, and yellow ink) are deposited is described below.
Suppose that the reflection density corresponding to the α wavelength range is OD_α(α) and the reflection density corresponding to the β wavelength range is OD_β(α) when only the ink α is deposited, and that the reflection density corresponding to the α wavelength range is OD_α(β) and the reflection density in the β wavelength range corresponding to OD_β(β) when only the ink of β is deposited.
In this case, it is assumed that the conditions of 0<OD_β(α)<<OD_α(α), and 0<OD_α(β)<<OD_β(β) are satisfied. It should be noted that an expression of “X<<Y” indicates that Y is much greater than X.
In an ideal color material (so called “block dye”), OD_β(α)=OD_α(β)=0 is satisfied. However, sub-absorption occurs in an actual color material, and thus OD_β(α) and OD_α(β) become larger than zero.
Suppose that the relation of OD_β(α)>OD_α(β) is satisfied, a case where the inks α and β are deposited so as to overlap with each other is described below.
When the ink α is first deposited and the ink β is subsequently deposited, then the ink β that has been deposited later has the characteristics of easily reflecting the light of the α wavelength range (i.e., the density corresponding to the a range is small) according to the relation of OD_β(α)>OD_α(β). Thus, as the ink β overlaps more, the lower the density of the ink α placed below β becomes.
Such phenomenon is especially conspicuous in a case in which pigment inks are used. Even when the same types of pigment inks are used, the rate of the above-mentioned reduction of the density changes as the particle diameter of the pigment changes. Further, the above-mentioned rate of reduction of the density changes because of the spectral characteristics of the covering power of the ink.
FIG. 22 shows a density measurement result of a sample of a single color of each of cyan and magenta, and that of a sample of the two colors that overlap with each other. The horizontal axis indicates the cyan (C) density and the vertical axis indicates the magenta (M) density. In this case, the color material M easily reflects light of the C wavelength range at the surface of the color material M. Thus, if the color inks C and M are overlapped in the order of C to M, then it can be observed that the density of the ink C is reduced more than that when only the ink C is deposited (see the curved arrow in FIG. 22).
This depends largely on the spectral characteristics of the color materials, and the above-described phenomena are not considered in either Japanese Patent Application Publication Nos. 62-161541 or 2003-112469.